The ferritic-austenitic stainless steels—the duplex steels—combine a high mechanical strength and toughness with good corrosion resistance, particularly as far as stress corrosion is concerned. For the corrosion resistance as well as for mechanical features such as weldability, it is important that the essential constituents of the steel, austenite and ferrite, are well balanced. In modern development of duplex steels, efforts are made to obtain a microstructure which contains 35-65% ferrite and 35-65% austenite. The duplex steels to an increased extent compete with traditional austenitic stainless steels within offshore, paper and pulp industry, chemical industry, and other fields where high strength and corrosion resistance are required. The duplex steels which so far are commercially available are, however, too expensive to find wider use, in spite of the fact that the duplex steels generally contain lower contents of the expensive alloy element nickel than comparable austenitic stainless steels.